Background music that embraces rather than eases tension isn't for everyone. Andre Custodio, a Pleasant Hill composer struggling to reach a small but appreciative audience with the spookily ethereal style known as "dark ambient," says people can get the wrong idea.

"I don't want to go so far as to say demonic, but that's sometimes how my work is described," said Custodio, who performs in San Francisco later this month with his wife, Phyll "Dark Muse" Smith, a vocalist and ambient artist in her own right. He'll also appear in March as part of a four-hour, nonstop drone-music fest at Oakland's 21 Grand. "It's not bubble-gummy in any way, shape or form," he said. "It's not very lighthearted."

Dark ambient is something like white noise turned inside out, with sinister droning and rumbling and textures of metal and dry percussion instead of high harmonies and the melodies of nature. Ground and sky give way to a galactic emptiness in which one floats homelessly. Lacking anything to hold on to, the listener has no recourse but to meditate upon the gloom.

"It's not pretty, but you'd need to redefine things to call it ugly," British reviewer Mick Mercer wrote in a 2002 look at "ten thousand things," the first album by Custodio's Nihil Communication project. "Because the only rules which apply here are your own. Can you take it, and having done so, use it?"

The ambient music label The Fossil Dungeon's Web site offers a pithier description of the style in a note on the music of Phyll Smith: raw emotion layered in sound to form a "sculpture of haunted space." Custodio, 35, stresses that the music holds up a mirror to the human condition, which he sees as combining the benevolent and the brutal. No one would call this a comforting vision. But in Custodio's view it's equally wrong to call it negative.

"No matter how you slice it, there are seeds of benevolence that each species on the planet has, but at the core of it we have to commit acts of violence to survive," he said. "We have to step on a bunch of ants in order to keep them out of the house. That's what life is.''

Custodio is influenced by the late Indian philosopher Jiddu Krishnamurti, who preached that it's necessary to sweep aside one's worldly attachments to see the truth.

But Custodio is no mystic. He makes music simply to convey sounds he hears in his head, not to send a message. A San Francisco native who works as a South of Market building manager, he comes across as a cheerful and rather playful young man who takes joy in experimentation.

For example, his solo performances as Nihil Communication center on a homemade instrument called a T-Rodimba. Designed by San Francisco instrument maker Tom Nunn, it's a plywood board fitted with industrial screws tuned to an eight-tone scale.

Like a marimba, the Rodimba was intended to be played with mallets. But Custodio likes to a use a bow, which creates screaming tones. "I usually take the experimental thing to the fullest extreme," said Custodio, who combines the Rodimba with synthesizers, percussion and vocals in his solo performances.

A formally trained percussionist, Custodio dropped out of mainstream music more than 10 years ago to pursue his love for sound design. He was inspired by the free-jazz Splatter Trio.

"The idea of jazz music set me free," he said. "I never really had personal experience in being an audience member to free jazz. When I saw what people could do with it, having experience and skill with your style, something snapped in me. I really had to dive in head-first."

He immersed himself in experimental music at a San Francisco distributor's warehouse and frequented a South of Market Goth club. He moved on from the Goth music scene when he felt its outré spirit waning.

Custodio is searching for an independent producer. Meanwhile, he moderates online communities at Myspace.com and Tribe.net, reaching out to listeners who find the chilly soundscapes of dark ambient to be strangely comforting.

"I'm not here to profess the apocalypse," he said. "There's a thing called the status quo, and there's us. That's the way I look at it. My duty as an artist and as a human being on the planet is to say, 'Hey, there's another way.' "

Contact Rick DelVecchio at rdelvecchio@sfchronicle.com